Magnetos!

The mags of ZS-SLM

With the tourist season in the stunning Western Cape behind us and all the fun flying with tourist and Hire & Fly pilots nearing an end, I’ve reflected on some of the experiences I’ve had.

I’ve noticed that magneto grounding checks are not done as often as they should be, and in some cases, it’s completely unheard of by some. But what is this magneto grounding check that we do? Why is it recommended? To understand the what’s & why’s, it is necessary to understand some basic knowledge of the magneto ignition system – It a system that works on the same principle as with all electric generators and alternators.

The technical stuff: If a magnet is spun past a coil at a sufficient rate, a current will be generated in said coil and it will flow through the circuit. The faster the magnet is spun past the coil, the higher the voltage generation will be. If sufficient voltage can be generated, that current can arc across two electrodes. A condenser, breaker points, a distributor, spark plug leads and spark plugs are connected to this coil; thereby forming a reliable and self-contained ignition system. Add another one to have two independent ignition systems, and you have a redundant ignition system for your aircraft! Awesome.

The magnetos are connected to the engine’s accessory case. This ensures that the magnetos will spin as long as the engine is running. Naturally, all of these wonderful things ensure that the engine will receive spark even if the plane’s electrical system were to fail. The only way to make sure the magnetos won’t produce a spark when the engine is turned off, is to ground them. This is done with the p-lead connected to the ignition switch. If that p-lead got disconnected somehow, the magnetos would be hot.

Risky business: Thankfully, most, if not all pilots know that it’s important to treat the propeller as if the magnetos were hot. What happens after a flight if the plane needs to be pushed into the hangar and the propeller stopped at an unwanted upright position? Are you going to trust that the magnetos are grounded and turn the propeller with your hand so that you can get the tow-bar connected? That would be considered by many a very unwise decision. Too many have suffered a serious injury from this non-commanded engine start. Follow your checklist for the correct shut-down procedure.

Safety & Prevention: This where the dead-cut check comes in. When our aircraft are flown in the training and club environment, a scenario presents itself where many different pilots and students are flying the same aircraft in a given day. Engines are started and shut down many times at our busy flight school. Different conditions are encountered by each of the pilots during the day. The possibility of a magneto grounding failure in between flights should not be discounted by anyone. For everyone to stay safe when handling the aircraft on the ground, the correct shutdown procedure must be followed.

The Left mag of ZS-SLM

Care: Since the avionics are very sensitive to voltage surges, they need to be disconnected. With the avionics master switch in the off position and the engine running at slow idle, switch the ignition switch to the off position. If the p-lead is properly connected, a positive, continual drop in engine RPM should be observed. Before the engine starts turning over too slowly, switch the ignition back to BOTH. This is to prevent kickback where the engine could start spinning in reverse. If a positive drop in RPM is not observed with the key in the OFF position, the magnetos are not being grounded and they remain hot. This leads to a definite unsafe condition where, if residual fuel is introduced into the combustion chamber when the propeller is turned over to attach the tow-bar, that the engine could start! To minimize the possibility of residual fuel finding its way into the engine, the engine is then shut down by setting the mixture control to the Idle-cutoff, or fully out position, thereby eliminating all fuel from the combustion chamber. Remove the key from the ignition and switch off the battery master switch.

Some more risk: There is one instance where a dead-cut check should not be performed. This is when the aircraft is in the vicinity of the fuel bay, because when the check is inadvertently done at a too high RPM, un-burnt air-fuel mixture escaping the exhaust can catch fire when ignition is introduced back to the engine. The dangers of that are not necessary to be expounded upon.

If these shutdown procedures are followed by all pilots, especially those flying shared aircraft at our flight school, an inadvertent engine start and injury will be prevented. It should go without saying, but, the procedure is not a replacement for common sense. When you go to move the propeller out of necessity, ensure that the keys are out of the ignition switch and in plain sight. As a last precaution when turning the prop on the ground: Have an open hand on the one prop blade and the other on the spinner. If the engine wants to fire up, simply push yourself away out of harm’s way.

I hope this might make you think about that aspect of flying again and increase your overall sense of safety. Thank you for your time and see you at the club soon!

Christo Smit
SFC Flight Instructor.